ABSTRACT: Multiple stressors in estuaries can cause declines in native species and impairment of ecosystem goods and services. In contrast, one stressorthe introduction of non-native speciesactually leads to higher local richness. We examined the changes in ecosystem function associated with introductions into Willapa Bay, Washington, USA, a relatively undeveloped estuary with 45 documented exotic marine species. The replacement of native oysters by 2 new bivalve species has increased secondary production of harvested suspension feeders by 250% over peak historic values (3.3 × 105 vs. 0.9 × 105 kg dry wt yr1), based on >150 yr of records of harvested biomass. Key aspects of aquacultureparticularly planted areahave remained constant over time, so we attribute much of the altered secondary production to higher growth rates of non-native species. The addition of 2 tracheophytes has increased primary production on the tideflats by >50% (5.3 × 107 vs. 3.5 × 107 kg dry wt yr1), which we calculated by scaling up local measurements of plant growth to the total area occupied by each species. These changes in production are also associated with altered detritus, water filtration, and biogenic habitat. Because other stressors are largely absent from Willapa Bay, the addition of particular exotic species has dramatically enhanced system production, while fundamentally reshaping the ecological character of the estuary. These strong ecological impacts of introduced species have rarely been measured at whole-ecosystem scales, and they occur in part because new species occupy habitats where similar native species were not present.